OT pour les Francaise: 3G pay-as-you-go in France?

[Ben Rubinstein]

I'm abusing the list to contact some of our French correspondents.  Apologies 
to everyone else... and apologies to French speakers for my appalling Franglaise.

I'm wondering whether it's possible to get hold of 3G data stick - or the SIM 
to go into one - for use in France, on a "pay-as-you-go" basis, rather than a 
long term contract.  In the UK, these 3G USB data modems, initially only 
available on quite long contracts with substantial monthly subscription, then 
became available under pay-as-you-go for about £100 (then about €130) and have 
since been dropping repeatedly so that you can now get one for about €25.

Theoretically the ones I have should work in France, but paying a very high 
premium over the domestica data rate - in practise I've never been able to 
make them work at all.  So I'm wondering about the possibility of getting a 
French one to use while I'm in France - but I'm there often enough/long enough 
to justify doing so on a contract.

I know that the mobile/cellular telephony market works very differently in 
different countries; are pay-as-you SIM cards (with 3G data access) or modem 
sticks available?  And are they truly pay-as-you-go, or (as seems to be the 
case in the US) somewhat lightweight contracts, where you have to top up by a 
minimum amount every few months otherwise it stops working altogether?

Any responses (off-list might be better!) gratefully received.

thanks,

- Ben